THE EVOLUTION OF THE MAMMALIA. 13 
the manner in which, and of the factors by which, that order has 
been brought about. 
At first, it seemed possible to accept the theory of evolution 
only as a general principle explanatory of broad classes of facts, 
and, in a very few instances, of concrete cases. It is one thing 
to be convinced that John Doe had ancestors living in the tenth 
century, but another and very different thing to determine just 
who those ancestors were. In the same way, we may be morally 
certain that a given animal is descended from a species which 
flourished in Eocene Tertiary times, without being at all able 
to point out the ancestral type, or to identify the successive steps 
of change which have led up to the modern form. 
The problems which the acceptance of the theory of evolution 
set before the naturalist were very many and very difficult, amount- 
ing in sum to the working out of the whole history of life. It is 
not to be expected that these problems can ever be solved in 
their entirety, but already a surprising amount has been accom- 
plished, and we cannot set any limit beyond which knowledge 
shall never be able to advance. Progress is slow but steady, and 
it is impossible to predict how far it may eventually lead us. 
A fundamental one of these problems, and one of the first to 
be attacked, was the construction of " genealogical trees " which 
should display the descent and mutual relationships of certain 
animal groups, after the analogy of the family trees that the gene- 
alogists make. Here, the most obvious difficulty (and one that 
has by no means been removed) was ignorance as to the possi- 
bilities of development and of the modes of its operation. No 
naturalist ever suggested that butterflies were derived from bees, 
or horses from dogs, but to this day there is little agreement as 
to the kind and the amount of change which may take place in 
any line of descent. Hence, the genealogical trees made by 
different investigators for the same group of animals differ from 
one another, and often radically, simply because there is no general 
consensus of opinion as to the manner in which evolutionary 
changes take place. What one writer regards as almost axiomatic, 
seems to another impossible and even absurd. 
